Retroreflective articles are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective articles in sheeting used in, for example, traffic signage and personal safety articles. Retroreflective sheeting is commonly employed in a variety of traffic control articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Cube corner sheeting, sometimes referred to as prismatic, microprismatic, triple mirror, or total internal reflection sheeting, is one type of retroreflective sheeting. This type of retroreflective sheeting typically includes a multitude of cube corner elements to retroreflect incident light. Retroreflective sheeting typically has a generally planar front surface and an array of cube corner elements protruding from the back surface. Cube corner reflecting elements include generally trihedral structures that have three approximately mutually perpendicular lateral faces meeting in a single corner—a cube corner. In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of intended observers and the light source. Light incident on the front surface enters the sheet and passes through the body of the sheet to be reflected by each of the three faces of the elements, so as to exit the front surface in a direction substantially parallel to the light source.
One method of manufacturing such prismatic cube corner sheeting is described in, for example, U.S. Pat. No. 3,689,346 (Rowland) and U.S. Pat. No. 5,691,846 (Benson et al). These patents generally describe methods of continuously replicating cube corner retroreflective articles by depositing a crosslinkable, partially polymerized resin on a negative molding surface to be replicated, and exposing the resin to actinic light or heat to solidify the resin. Because the materials are deposited and then cured, the process is often referred to as “cast and cure.” A disadvantage of this manufacturing method is that the resulting sheeting has cube corner elements that exhibit relatively high levels of shrinkage upon solidifying or curing, thus giving rise to optical imperfections in the cube corner microstructure, i.e., changes in the angles between the faces of the cube corner that produce light scattering rather than the desired maximum retroreflectivity. If the angles between faces of a replicated cube corner element cannot be controlled and maintained (e.g., because of shrinkage effects, distortion upon removal from the mold, or distortion due to thermal or mechanical stresses), the efficiency of retroreflection will be materially affected. Even a slight lack of control and maintenance of the cube corner geometry can significantly adversely affect the resultant retroreflective efficiency.
U.S. Pat. No. 5,988,820 (Huang et al) describes high elastic modulus cubes and a low elastic modulus body layer. This combination is advantageous because the sheeting can be highly flexed without suffering from a substantial loss of retroreflectivity. The cube corner elements demonstrate extraordinary dimensional stability during flexing and thereby provide good retroreflective performance when conformed. The dimensional stability and good retroreflective performance can be maintained at high temperatures. However, use of a low elastic modulus body layer requires the use of a carrier (e.g., a polyester carrier) to permit cast and cure processing. Without the carrier, the low elastic modulus body layer will stretch too much during the winding and unwinding processes.
U.S. Pat. No. 6,325,515 (Coderre et al) and U.S. Pat. No. 7,329,447 (Chirhart et al) describe a “bilayer” overlay layer including PET and COPET. The overlay layer of this patent protects the underlying optical structures (e.g., cube corner elements) from the environment.